This disclosure is related to the following simultaneously-filed disclosures that are incorporated herein by reference:
Coherent Population Trapping-Based Method for Generating a Frequency Standard Having a Reduced Magnitude of Total a.c. Stark Shift of inventors Miao Zhu and Leonard S. Cutler Ser. No. 09/588,045;
Coherent Population Trapping-Based Frequency Standard Having a Reduced Magnitude of Total a.c. Stark Shift of inventors Miao Zhu and Leonard S. Cutler Ser. No. 09/587,719; and
Coherent Population Trapping-Based Frequency Standard and Method for Generating a Frequency Standard Incorporating a Quantum Absorber that Generates the CPT State with High Efficiency of inventor Miao Zhu Ser. No. 09/587,717.
The invention relates to high-precision instruments, such as frequency standards, magnetometers and laser spectrometers, that detect a resonant interaction between incident electro-magnetic radiation and a quantum absorber, and in particular relates to a detection method, a detector and a high-precision instrument in which a high signal-to-noise ratio detection signal is generated that quantifies the resonant interaction between the quantum absorber and the incident radiation.
High-precision instruments, for example, frequency standards, magnetometers and laser spectrometers, are known in the art. Such instruments generate an electronic detection signal that quantifies a resonant interaction between incident electro-magnetic radiation and a quantum absorber. The detection signal is then used to control, or to enable measurement of, a characteristic of the incident electro-magnetic radiation, such as the frequency of a frequency component of the electro-magnetic radiation or an external magnetic field.
In such precision instruments, the quantum absorber is irradiated with the incident electro-magnetic radiation from by a suitable source. The quantum absorber in a lower quantum state can absorb one or more photons from the incident electro-magnetic radiation, and moves to an upper quantum state. This absorption process decreases the intensity of the electro-magnetic radiation transmitted by the quantum absorber. When the quantum absorber in the upper quantum state decays spontaneously to the lower quantum state, it emits one or more photons of fluorescent electro-magnetic radiation. The rate of the absorption and re-emission process depends on a number of conditions in the photon-quantum absorber interaction.
For a specific application, the rate of the absorption and re-emission process is designed to depend on a specific resonance condition, such as an optical resonance condition, that is satisfied when the frequency of the incident electro-magnetic radiation, or a frequency component thereof, equals the transition frequency of the quantum absorber. When this resonance condition is satisfied, the rate of the absorption and re-emission process changes. Consequently, the resonance condition can decrease the transmitted electro-magnetic radiation and increase the fluorescent electro-magnetic radiation, or vice versa.
The resonance condition is conventionally detected by detecting only the transmitted electro-magnetic radiation or by detecting only the fluorescent electro-magnetic radiation. In such detection schemes, the quantification of the resonance condition is usually limited by the signal-to-noise ratio of the electrical signal generated by detecting only the transmitted electro-magnetic radiation or by detecting only the fluorescent electro-magnetic radiation. The limitation on the quantification of the resonance condition limits the stability and accuracy of the high-precision instrument that employs it.
Thus, what is needed is a detection method and detector that generate a detection signal having as large a signal-to-noise ratio as possible. Such a detection method and detector will increase the accuracy and stability of any high-precision instrument whose accuracy and stability was formerly limited by the signal-to-noise ratio of the detection signal.
The invention provides a detection method, detector and high-precision instrument in which both the transmitted electro-magnetic radiation and the fluorescent electro-magnetic radiation are detected to generate respective electrical signals, and in which the electrical signals are combined with an optimized relative weighting to generate the detection signal. The detection signal has a signal-to-noise ratio greater than the signal-to-noise ratios of the electrical signals generated by detecting the transmitted electro-magnetic radiation alone or by detecting the fluorescent electro-magnetic radiation alone. The improved signal-to-noise ratio of the detection signal enables the detection signal to provide a more accurate and stable quantification of the resonance condition of interest.
Specifically, the invention provides a detection method for generating a detection signal that quantifies a resonant interaction between a quantum absorber and incident electro-magnetic radiation. In the method, the quantum absorber is irradiated with the incident electro-magnetic radiation. The quantum absorber absorbs a portion of the incident electro-magnetic radiation and generates fluorescent electro-magnetic radiation in response to it. The quantum absorber additionally transmits the unabsorbed portion of the incident electro-magnetic radiation. The unabsorbed portion of the incident electro-magnetic radiation is detected to generate a first signal that has a first signal-to-noise ratio. The fluorescent electro-magnetic radiation is detected to generate a second signal that has a second signal-to-noise ratio. The first signal and the second signal are combined to generate the detection signal. The detection signal has a signal-to-noise ratio greater than the first signal-to-noise ratio and the second signal-to-noise ratio.
The invention also provides a detector for generating a detection signal that quantifies a resonant interaction between a quantum absorber and incident electro-magnetic radiation. The detector comprises a first detector, a second detector and a combiner. The first detector is located to receive a portion of the incident electro-magnetic radiation that remains unabsorbed by the quantum absorber, and operates to generate a first signal in response to the unabsorbed portion. The first signal has a first signal-to-noise ratio. The second detector is located to receive fluorescent electro-magnetic radiation generated by the quantum absorber in response to the incident electro-magnetic radiation, and operates to generate a second signal in response to the fluorescent electro-magnetic radiation. The second signal has a second signal-to-noise ratio. The combiner is connected to receive the first signal and the second signal and operates to generate the detection signal from the first and second electrical signals. The detection signal has a signal-to-noise ratio greater than the first signal-to-noise ratio and the second signal-to-noise ratio.
Finally, the invention provides a precision instrument that comprises a source of incident electro-magnetic radiation, a quantum absorber, a first detector, a second detector and a combiner. The quantum absorber is located to receive the incident electro-magnetic radiation from the source. The quantum absorber absorbs a portion of the incident electro-magnetic radiation and generates fluorescent electro-magnetic radiation in response to the incident electro-magnetic radiation. The quantum absorber additionally transmits an unabsorbed portion of the incident electro-magnetic radiation. The first detector is located to receive the unabsorbed portion of the incident electro-magnetic radiation, and operates to generate a first signal in response to the unabsorbed portion. The first signal has a first signal-to-noise ratio. The second detector is located to receive the fluorescent electro-magnetic radiation, and operates to generate a second signal in response to the fluorescent electro-magnetic radiation. The second signal has a second signal-to-noise ratio. The combiner is connected to receive the first signal and the second signal, and operates to generate a detection signal from the first and second electrical signals. The detection signal has a signal-to-noise ratio greater than the first signal-to-noise ratio and the second signal-to-noise ratio.